Toxic and heavy metals are one of the most problematic classes of contaminants due to their ubiquity and toxicity. Heavy metals represent a significant source of pollution when released into the environment. They are present in fossil fuels and ores, for example, and are released into the environment via airborne emissions during industrial processing of these materials, e.g., during incineration, or leach into soils and groundwater from ash and other residues when these materials are landfilled. Heavy metals from all sources present a major environmental concern.
Heavy metals are present in fossil fuels such as coal, oil and natural gas, in biomass, in ores and in wastes. Heavy metals are volatilized in the hot regions of process units such as boilers, incinerators or furnaces used for waste disposal, energy generation or metal recovery. Subsequently, as the gases are cooled, less volatile metal species (e.g., cadmium and lead) condense onto particles of ash entrained in the gas stream, while more volatile metals (e.g., arsenic and mercury) remain in the gas phase, where they end up as airborne emissions.
Heavy metals include, for example, arsenic, beryllium, lead, cadmium, chromium, nickel, zinc, mercury and barium. Most of these metals are highly toxic to humans and animals. Metal-contaminated wastes often also contain organic contaminants. Thus, treatment technologies for treating wastes contaminated with toxic metals preferably should be effective for treating organic waste as well.
Toxic metals may enter a combustion system in many physical and chemical forms, for example, as constituents of a hazardous or municipal solid waste to be incinerated or as trace quantities in coal. Once introduced into a combustion environment, a metal may undergo transformations to different phases as well as to different chemical species depending upon combustion conditions and the presence of chlorine and other reactive species. Also, at combustion temperatures, metals may be vaporized and then undergo nucleation to form a submicron aerosol, or the metal vapor may condense onto existing particles. These resulting primary particles, formed by nucleation or condensation, have been observed to have a diameter of approximately 0.02 Fm. Through growth by condensation of vapor or by coagulation with other particles, these particles ultimately may have a diameter of from about 0.02 Fm to about 1.0 Fm in the flue gas. For example, in one study on hospital waste incineration, a bimodal distribution in the flue gas was observed, and the particles having a diameter between about 0.1 Fm and 0.2 Fm accounted for 7% to 74% of the lead, 62%-77% of the cadmium, and 20%-80% of the zinc in the total particulate phase. (Kauppinen, E. l. and Pakkanen, T. A., "Mass and Trace Element Size Distributions of Aerosols Emitted by a Hospital Refuse Incinerator", Atmos. Environ., 24A, 423 (1990).
Unfortunately, flue gas cleaning equipment used in combustion systems is least efficient in capturing particles having diameters in the submicrometer size range. For example, electrostatic precipitators are used in many coal-fired combusters and typically exhibit the lowest collection efficiency for particles less than 1 Fm in diameter. Particles in these size ranges potentially pose a greater health threat than larger particles since they penetrate deeper into the lungs where the toxic materials come into contact with the blood. This potential adverse health impact of metal emissions from combustion devices is an appropriate incentive to investigate new methods and technologies for metal removal from waste gas streams. Furthermore, the United States Environmental Protection Agency (US EPA) has begun to regulate toxic metal emissions from combusters pursuant to Title III of the 1990 Clean Air Act Amendments which specifically lists eleven metals and their compounds as air toxics.
In an attempt to control such toxic metal emissions, researchers have proposed several control methods using various bulk solid sorbents to chemically adsorb various metals thereby reducing their discharge in particulate form into the atmosphere.
One such method includes combusting a metal contaminated waste in a fluidized bed of sorbent. (Ho, T., Chen, J., Hopper, J. and Oberacker, D., "Metal Capture During Fluidized Bed Incineration of Wastes Contaminated with Lead Chloride", Combust. Sci. and Technol., 85, 101 (1992)). Other proposed methods include injecting a sorbent into the high temperature region of a combustion device (Scotto, M., Petersen, T. and Wendt, J., "Hazardous Waste Incineration: The In-Situ Capture of Lead by Sorbents in a Laboratory Down-Flow Combuster", 24th International/Symposium on Combustion, the University of Sydney, Sydney, Australia (1992), and passing a metal vapor at high temperatures through a packed bed of sorbent (Uberoi, M. and Schadman, F., "High Temperature Removal of Cadmium Compounds Using Solid Sorbents", Environ. Sci. Technol., 25, 7, 1285 (1991); and Uberoi, M. and Schadman, F., "Sorbents for Removal of Lead Compounds from Hot Flue Gases", AlChE Journal, 36, 2, 307 (1990)). However, none of these methods adequately control the emission of mercury.
Mercury emissions from combustion sources have been a great concern (Chu, P. and Porcella, D. B. Water, Air, Soil Pollut. 1995, 80, 135-144; Krishnan, S. V., Gullett, B. K. and Jozewicz, W. Environ. Sci. Technol. 1994, 2(Y((Y), 1506-1512). Unlike most other heavy metals that are emitted in particulate forms, mercury has been reported to be released mainly in the elemental form in the vapor phase. Data for waste incinerators show the fraction from 10% to 90% depending on the waste composition and operating conditions (Lindqvist, O. Waste Management & Research. 1986, 4, 35-44; Bergstrom, J. G. T. Waste Management & Research, 1986, 4, 57-64; Reimann, D. O. Waste Management & Research, 1986, 4, 45-56; Hall, B., Schager, P. and Lindqvist, O. Water, Air Soil Pollut., 1991, 56, 3-14; Livengood, C. D., Huang, H. S., Mendelsohn, M. H. and Wu, J. M. PETC=s 10th Annual Coal Precapture, Utilization and Environmental Control Contractors Conf., July 1994). Data for coal-fired power plants show a higher fraction, in some cases even over 95% (Meij, R. Water, Air, Soil Pollut. 1991, 56, 21-33; Larjava, K., Laitinen, T., Vahlman, T., ArtMann, S., Siemens, V., Broekaert, J. A. C. and Klockow, D. Intern. J Environ. Anal. Chem., 1992, 49, 73-85; Morency, J. R. PETC=s 10th Annual Coal Precapture, Utilization and Environmental Control Contractors Conference, July 1994, Vogg, H., Braun, H., Metzger, M. and Schneider, J. Waste Management & Research, 1986, 4, 65-74). Vapor phase elemental mercury is negligibly captured in typical air pollution control devices. Once emitted into the atmosphere, mercury may undergo various biological processes in the atmosphere to form even more toxic mercury species such as methyl mercury. It may also bioconcentrate in vegetation and fish. The consumption of these produce and fish leads to adverse health effects in human beings and predator animals (Seigneur, C., Wrobel, J. and Constantinou, E. Environ. Sci. Technol., 1994, 28(9), 1589-1597; Hall, B., Lindqvist, O. and Ljungstrom, E. Environ. Sci. Technol. 1990, 24(1), 108-111; Aizpun, B., Fernandez, M. L., Blanco, E. and Sanz-Medel, A. J. Anal. At. Spectrom. 1994, 9, 1279-1284). Due to its high toxicity, stringent regulations have been set for mercury emissions (Quimhy, J. M. 86th Annual Meeting of Air & Waste Management Assoc., Paper 93-MP5.03, Denver, Colo., Jun. 13-18, 1993; 40 CFR 266 (1992); Clarke, M. J. 86th Annual Meeting of Air & Waste Management Assoc., Paper 93-RP 154.01, Denver, Colo., Jun. 13-18, 1993; Linak, W. P. and Wendt, J. O. L. Prog. Energy Combust. Sci., 1993, 19, 145-185; Trichon, M. and Chang, R Proc. 1992 Incin. Conf., Albuquerque, N. Mex., 1992, 255-259), and effective control of mercury emission is extremely important.
Environmental standards for particulate and total mercury emissions by coal-fired power plants, petroleum refineries, chemical refineries, incinerators, metallurgical operations, thermal treatment units, and other particulate- and mercury-emitting facilities are becoming increasingly more demanding. New regulations are currently under development not only to amend existing regulations to reduce further permissible levels of total mercury emissions for such facilities but also to regulate total mercury emissions from a wide variety of other types of operations not presently subject to such regulations.
To effectively control mercury emissions, understanding the behavior of mercury in the combustion environment is very important where speciation plays a critical role (Galbreath, K. C. and Zygarlicke, C. J. Environ. Sci. Technol., 1996, 30(8), 2421-2426). Different species possess different chemical and physical properties that result in different fates. For example, mercury chloride is soluble and may be washed out by gas washing devices such as scrubbers. This has been reasoned for the cause of the higher capture efficiencies in certain waste incinerators because chlorine content is typically high in the waste (Lancia, A., Musmarra, D., Pepe, F. and Volpicefli, G. Combust. Sci. Tech., 1993, 93, 277 289; Peterson, J., Seeger, D., Skarupa, R., Stohs, M. and Hargrove, B. and Owens, D. 87th Annual Meeting of Air & Waste Management Assoc., Paper 94-RP114B.01, Cincinnati, Ohio, Jun. 19-24, 1994). Mercury oxide, on the other hand, is less volatile and may form particles. Thus, it can be potentially captured in particulate control devices. As oxidized forms of mercury can be more easily removed from the gas stream than the elemental mercury, they have been proposed as the preferred forms to be generated in combustion facilities (Wu, C. Y., Arar, E. and Biswas, P. 89th Annual Meeting of the Air & Waste Management Assoc., Nashville, Tenn., Jun. 23-28, 1996; Helfritch, D., Harmon, G. and Feldman, P. 89th Annual Meeting of Air & Waste Management Assoc., Paper 96-ES96.41, Nashville, Tenn., Jun. 23-28, 1996). However, a Hg atom has a 5d.sup.10 6s.sup.2 closed shell electronic structure which is isoelectronic to He (ls.sup.2) (Haberland, H., von Issendorff, B., Yufeng, J., Kolar, T. and Thanner, G. Z. Phys. D., 1993, 26, 8-12; Brechignac, C., Broyer, M., Cahuzac, Ph., Delacretaz, G., Labastie, P., Wolf, J. P. and Woste, L. Pllys. Rev. Lett., 1988, 60(4), 275-278; Uchtmann, H., Rademann, K. and Hensel, F. Ann. Physik Leipzig, 1991, 48(1-3), 207-214). This structure results in its unusual nonreactivity among metals. Past studies have confirmed this point by showing extremely slow or no oxidation in air at high temperatures. Other studies show that oxidation occurs only with strong oxidants such as NO.sub.2, HCl or Cl.sub.2 (Hall, B., Schager, P. and Lindqvist, O. Water, Air Soil Pollut., 1991, 56, 3-14; Hall, B., Lindqvist, O. and Ljungstrom, E. Environ. Sci. Technol. 1990, 24(1), 108-111; Hall, B., Schager, P. and Ljungstrom, E. Water, Air Soil Pollut., 1995, 81, 121-134). Nevertheless, these reactants can not be used to control mercury emissions since they themselves are also regulated pollutants.
A lot of studies to capture elemental mercury by other approaches have also been conducted. Currently, the mostly widely used technique is activated carbon, and activated carbon impregnated with sulfur, chlorine or iodine is found to be the most effective. However, the use of activated carbon is limited because of its poor capacity, low applicable temperature range, regeneration and slow adsorption rate (Otani, Y., Kanaoka, C., Usul, C., Matsui, S. and Emi, H. Environ. Sci. & Technol., 1986, 20(7), 735-738). The other technique proposed is corona discharge (Urabe, T., Wu. Y., Nagawa, T. and Masuda, S. Seiso Giho, 1988, 13, 12-29). Radicals such as OH, O and O.sub.3 are generated in such an environment which then oxidize elemental mercury. Very high efficiency of oxidation has been demonstrated by using pulsed, positive voltage corona with energy density 10 w/cfm (Helfiitch, D., Harmon, G. and Feldman, P. 89th Annual Meeting of Air & Waste Management Assoc., Paper 96-ES96.41, Nashville, Tenn., Jun. 23-28, 1996). However, no data of the particle size distribution resulting from the subsequent nucleation of mercury oxide particles are available. Typically, metallic particles formed by nucleation in combustion environments are enriched in the submicrometer regime (Linak, W. P. and Wendt, J. O. L. Prog. Energy Combust. Sci., 1993, 19, 145-185), and it is well known that typical particulate control devices have a minimum of collection efficiency in this regime (Flagan, R. C. and Seinfeld, J. H. 1988, Fundamentals of Air Pollution Engineering, Prentice Hall, Englewood Cliffs, N.J.). Therefore, alternative approaches need to be developed.
Electrostatic precipitators and filters are two common particulate removal systems for removing particulate material from a gas stream. In electrostatic precipitators, electrodes impart a negative electrical charge to the particulate material. The charged particulate material migrates to positively charged collection plates where the material is collected. The collected particulate material is periodically removed from the collection plates for disposal.
However, electrostatic precipitators are unable to remove effectively particulate material having inadequate resistivity to retain an electrical charge. For particulate material that can retain an electrical charge, the particle collection efficiency of electrostatic precipitators has been found to decrease over time. It is believed that the decrease is primarily a result of electrical spark over from the collection plates to the electrodes.
While the above-described particulate removal systems remove particulate material, neither electrostatic precipitators nor filters remove vaporous forms of mercury from the gas stream or these systems have a low efficiency of capture of submicrometer sized particles. This necessitates modification of the systems for mercury removal. In particular in coal-fired power plants, a variety of sorbents are often introduced upstream of the filter to remove mercury from the flue gas. The sorbents are recovered by the filter as part of the collected particulate material layer.
The efficiency of sorbents frequently utilized for mercury removal in facilities of the above-noted nature are often inadequate for meeting the increasing regulatory demands concerning total mercury emissions. Additionally, such sorbents typically do not remove all forms of mercury. Since certain types of fuels or raw materials produce only certain forms of mercury when consumed, it is therefore often necessary to base the selection of the specific sorbent to be used upon the chemical composition of the fuel or raw material to be consumed in the operation.
Sorbent particles have been demonstrated to be effective to capture certain toxic metals in combustion environments (Uberoi, M. and Shadman, F. AlChE Journal, 1990, 36(2), 307-309; Uberoi, M. and Shadman, F. Ind Eng. Chem. Res., 1991, 30, 624-631; Ho, T. C., Chen, C., Hopper, J. R. and Oberacker, D. A. Combust. Sci. Tech., 1992, 85, 101). Compared with traditional bulk sorbent particles used in a fixed bed or fluidized bed, in-situ generated particles have been shown to possess a higher capture efficiency (Owens, T. M. and Biswas, P. Ind. Eng. Chem. Res. 1996, 35(3), 792-798; Owens, T. M. and Biswas, P. J. Air Waste Manage. Assoc., 1996, 46, 530-538) as well as to suppress the formation of submicrometer particles (Gulett, B. K. and Ragnunathan, K. Energy & Fuels, 1994, 8, 1068-1076). It is known that radicals are generated on titania surface when ultraviolet (UV) radiation is applied, and these radicals can be used to convert chemicals. Bulk solid sorbents have been widely used to remove organic pollutants (Jacobsen, A. E. Ind Eng. Chem. 1949, 41, 523) or heavy metals in waste water (Aguado, M. A., Gimenez, J. and Cervera-March, S. Chem. Eng. Comm., 1991, 104, 71-85). Kaluza and Boehm (Kaluza, U. and Boehm, H. P. J. Catal. 1971, 22, 347) applied titania as a thin film on a glass slide with a drop of mercury. Yellow color (representing oxide formation) was observed after 1.5 hr with UV in the range of 410-390 nm at 60.degree. C. They reasoned the oxidation was due to the OH groups on titania surface under the presence of oxygen.
Additionally, in an attempt to control emissions of toxic organic compounds, many researchers have utilized irradiation to oxidize the organic compounds to less toxic or nontoxic products. U.S. Pat. No. 5,417,825, Graham, issued May 23, 1995, provides a method and apparatus for photothermal detoxification of toxic organic compound. The apparatus includes a thermally insulated reaction vessel maintained at a temperature greater than 200.degree. C., and a radiation source such as arc emission or a laser which irradiates the compound at a wavelength of less than 300 nm for at least 2 seconds to produce nontoxic reaction products. While this patent discloses the breakdown of a wide variety of toxic organic compounds, the reference does not mention the use of radiation to eliminate mercury vapors.
U.S. Pat. No. 4,210,503, Confer, uses ultraviolet lamps to irradiate a gas stream containing vinyl chloride, forming less hazardous materials which are then absorbed in a scrubber. Legan, U.S. Pat. No. 4,045,316, discloses a process for decontaminating gaseous or vaporous streams of vinyl chloride in which the streams are exposed to heat and radiation.
U.S. Pat. No. 5,449,443, issued Sep. 12, 1995, discloses a method whereby organic pollutants and bioaerosols in a gaseous stream are oxidized by exposure to light (e.g., UV light) in the presence of semiconductor catalyst particles or coatings supported on flexible strips suspended in the gaseous stream.
U.S. Pat. Nos. 4,888,101 and 5,118,422, Cooper, describe a system for photocatalytically modifying a chemical composition and water purification. The system includes semiconductor powder dispersed and trapped in a layer of glass wool. U.S. Pat. Nos. 4,892,712 and 5,032,241, Robertson, describe a device for purifying water or air. The device includes a high surface area matrix with photoreactive metal semiconductor bonded to the matrix. U.S. Pat. No. 4,943,357, Van Antwerp, describes a method of photodegrading a metallic chelate complex in an aqueous solution. There is no disclosure of a method for removing organic contaminants from air. U.S. Pat. No. 5,035,784, Anderson, describes a process for degrading complex organic molecules by photocatalysis. U.S. Pat. No. 5,126,111, Al-Ekabi, describes a method of removing, reducing, or detoxifying organic pollutants from water or air with a photoreactive metal semiconductor material, while simultaneously contacting the photoreactive material with a substance that accepts electrons. U.S. Pat. No. 5,144,146, Wekhof, describes a method for destroying toxic substances using UV radiation. No description is made of a photocatalyst. U.S. Pat. No. 5,174,877, Cooper, describes apparatus for photocatalytic treatment of liquids. No description is made of treatment of air.
U.S. Pat. No. 4,416,748, Stevens, issued Nov. 22, 1983, discloses that the content of SO.sub.2 and/or NO.sub.[x] in flue gas is reduced by irradiating the flue gas in admixture with NH.sub.3 with ultraviolet at wavelengths between 170 and 220 nm. The present invention relates to a process for the reduction of the content of SO.sub.2 and/or the nitrogen oxides NO and NO.sub.2 (sometimes referred to by the general term "NO.sub.[x] ") in flue gases.
U.S. Pat. No. 5,397,552, Weighold et al., issued Mar. 14, 1995, discloses a method of photochemically oxidizing gaseous organic compounds, an apparatus for photochemically oxidizing gaseous organic compounds, a lining insert for an apparatus for photochemically oxidizing gaseous organic compounds, and a method for producing such a lining. The principal method comprises: a) exposing gaseous organic compounds to ultraviolet light to oxidize the gaseous organic compounds into gaseous oxidation products; and b) reacting the gaseous oxidation products with an internal surface of a reaction chamber, the internal surface comprising a dry porous cementitious and chemically sorbent material which is chemically reactive with the gaseous oxidation products, with the gaseous oxidation products being reacted with the dry porous cementitious material to produce solid reaction products incorporated in sidewalls of the chamber.
U.S. Pat. No. 5,556,447, Srinivasachar et al., issued Sep. 17, 1996, describes a process for treating wastes contaminated by toxic metals and/or organic materials. The process involves heating the metal-contaminated wastes to a temperature sufficient to volatilize the metals. This temperature is also high enough to destroy or volatilize organic contaminants. The metal vapors are contacted with a sorbent which is reactive with the metals and sequesters them, thereby forming a non-leachable complex which can be disposed as non-hazardous conventional waste.
The art has lacked a relatively simple, highly efficient method for effectively removing mercury vapors from an exhaust in a continuous process. Therefore, it would be extremely desirable to have a method of reducing toxic metal emissions from a combustion system in which the system removes mercury more efficiently, thereby providing enhanced capture and subsequent storage of toxic metals at a reduced cost. The invention disclosed and claimed herein achieves these advantages in a manner not disclosed or suggested by the prior art.